The more interesting question might be what percent of senior employees would quit Google if ordered to move. I have no direct evidence, but my sense is potentially much higher than the average: they could care less about living costs, don't want to uproot their families, love CA weather, love living in such a tech heavy area, have access to better schools than in Kansas, etc.

FWIW, my old company added a Seattle office a few years ago. Sure, some people moved, but it was pretty negligible.